submarine

Why not deal with it the same way as most other MMORPGs deal with it. When you want to log out from the server your character sits down for a specific amount of time and after that time has passed you get logged off the server and your character dissapears immediately from the server. It could also be possible to vary the timer depending on if there's combat nearby etc. If you crash / alt-f4 then your character will remain in game for a specific amount of time as well but the amount could be higher than if you log out the "normal" way. If your character stays on even though you log off normally that means that players may find you by accident and kill you while your logged off, and I don't think that's good design.